For centuries, it was known that people who survived smallpox became immune to it. For that reason, nearly every culture tried to induce immunity in healthy individuals. The Chinese used tubes to insert powdered smallpox scabs into their nostrils. In Turkey, pus from lesions was scratched into the skin.
Climate graph San Francisco. The driest month is July. There is 1 mm of precipitation in July. Most precipitation falls in January, with an average of 115 mm. Temperature graph San Francisco. With an average of 17.6 Â°C, September is the warmest month. In January, the average temperature is 10.4 Â°C. It is the lowest average temperature of the whole year. San Francisco Climate table // historical Weather data
In South San Francisco, the average percentage of the sky covered by clouds experiences significant seasonal variation over the course of the year. The clearer part of the year in South San Francisco begins around May 23 and lasts for 5.0 months, ending around October 24. On July 20, the clearest day of the year, the sky is clear, mostly clear, or partly cloudy 92% of the time, and overcast or mostly cloudy 8% of the time.